In the petrochemical and other industries, valves are used on a wide scale basis to control the flow of process fluids. Due to the number of process valves in use at many modern industrial facilities, and the relatively large number of persons at those facilities that open and close one or more of the valves, or that may have the need or occasion to do so, it inevitable that workers will mistakenly open or closed a valve for a particular process line on occasion in the mistaken belief that they are opening or closing a valve for a different process line. Moreover, in many of these facilities, the process fluid controlled by a valve may be discharged at a location substantially downstream from the location at which the valve is located, and a mistaken valve operation is not always immediately apparent to the person operating the valve.
Since the inadvertent or mistaken opening or closing of a valve may result in personal or economic injury in some situations, it is frequently desirable, and sometimes mandatory, to lock a process line valve in a predetermined position. This predetermined position is usually either in a fully opened or fully closed position. The need to prevent mistaken or inadvertent valve operation is particularly acute when the process fluids controlled by the valve are hazardous. Indeed, when numerous hazardous process fluids are processed in large complex facilities, the locking of a valve in a predetermined position, e.g., a fully closed position, may be the most or only practical way of preventing a worker from mistakenly initiating flow of a hazardous process fluid in an untimely manner, and possibly causing injury to another worker downstream of the valve.
There have been numerous attempts to use lockout devices in the prior art to prevent the unintentional or mistaken opening or closing of a valve. For the most part, these lockout devices have successfully achieved the purpose of preventing the inadvertent or mistaken valve operation. However, the use of many of the prior art lockout designs have resulted in other problems. For example, one prior art valve lockout design for a plug valve use immovably secures a first stationary structural member with an outwardly extending stationary flange between the valve body and the valve cover. A second movable structural member then is rotatably attached to the valve shaft. Each of the first and second structural members is provided with an aperture that is alignable with the aperture of the other structural member when the shaft is rotated to a certain predetermined angular position, as for example to a closed position of the valve. A lock is then placed through the two aligned apertures preventing relative movement between the stationary and rotatable members, thus preventing rotation of the valve shaft relative to the stationary valve body. Installation of the above described first structural members, however, requires removal of the valve cover, and removal of the valve cover requires shutdown of any pressurized process line being controlled by the valve, and disturbance of the pressure boundary established by the valve cover.
Another prior art design that has achieved some success for use on plug valves includes a bracket that is mounted for common rotation with the valve shaft. The bracket has outwardly extending lugs that are positioned to engage upstanding bosses on a plug adjustment plate when the shaft is moved to a predetermined position. The engagement between the lugs and the bosses limit additional movement of the shaft, and the valve is thus prevented from moving to a prohibited position. However, the adjustment plate is used for thrust bolts, which thrust bolts are, in turn, used to adjust the pressure on the diaphragm in the valve. Hence, while such a design generally works satisfactorily, it suffers from the limitation that excessive torque applied to the valve shaft tends to move the thrust bolts. Movement of the thrust bolts, of course, is disadvantageous in that such movement may adversely affect the integrity of the diaphragm seal located at the top of the plug member.